Molten salt electrolytes are employed for ion transport in fuel cells for the generation of electricity. See, for example, U.S. Pat. Nos. 4,480,017, 4,410,607 and 4,079,171. Molten carbonate electrolytes have been used extensively in fuel cell applications. See, for example, U.S. Pat. Nos. 5,354,627 and 5,989,740. Molten salt fuel cells can be supplied with reformed fuel gas from an external reformer system. Alternatively, molten salt fuel cells can incorporate an internal reforming catalyst, i.e., a steam reforming catalyst, to produce hydrogen-containing gas (e.g. synthesis gasp for use at the gas electrode side of the fuel cell for generation of electricity. See, for example, U.S. Pat. Nos. 5,075,277; 5,380,600; 5,622,790; and 6,090,312 for internal reforming fuel cells.
Molten salt electrolytes have also been employed in electrochemical cells for gas separation, e.g., for the separation of oxygen, via transport of oxygen-containing anions through the molten salt. See, U.S. Pat. No. 4,859,296. More specifically, molten nitrate salt electrolyte has been employed in an electrochemical cell for oxygen separation via transport of nitrate ion (U.S. Pat. No. 4,738,760).
Molten chloride salt electrolytes (lithium chloride and potassium chloride) have been employed in electrochemical cells for the recovery of chlorine from hydrogen chloride gas. See, Yoshizawa, S. Et al. (1971) J. Appl. Electrochem. 245-251. U.S. Pat. Nos. 5,618,405 and 5,928,489 report the removal and recovery of hydrogen halides from gas mixtures using molten halide salt electrolytes.
In electrochemical and fuel cells employing molten salt electrolytes, a porous electrolyte plate (or tile) is made from a porous non-conducting matrix impregnated with the molten salt and positioned between an anode and a cathode. The porous matrix is typically made of a refractory, non-electron-conducting, inorganic material, such as lithium aluminate or lithium titanate. The electrolyte plate conducts or mediates ions between the anode and the cathode via the molten salt. The molten salt is selected for transport or mediation of a desired ion, e.g., a carbonate salt is used for mediation of a carbonate anion or a chloride salt is used for transport of chloride ion. The molten salt electrolyte plate does not conduct electrons. The anode and cathode of the electrochemical or fuel cells are electrically connected through an external circuit for electron transport.
Catalytic membrane reactors using gas-impermeable solid state membranes for the oxidation or decomposition of various chemical species have been extensively studied. One potentially valuable use of such reactors is in the production of synthesis gas. See, for example, Cable et al. EP patent application 90305684.4 (published Nov. 28, 1990) and Mazanec et al. U.S. Pat. No. 5,306,411. Synthesis gas, a mixture of CO and H2, is widely used as a feedstock in the chemical industry for production of bulk chemicals such as methanol and liquid fuel oxygenates.
Catalytic membrane reactors can also be employed for steam reforming of hydrocarbons. Steam reforming involves the following reactions illustrated with methane as the hydrocarbon:
CH4+H2Oxe2x80x94xe2x86x92CO+2H2
CH4+2H2Oxe2x80x94xe2x86x92CO2+4H2
CO+H2Oxe2x80x94xe2x86x92CO2+H2(Water gas shift reaction).
U.S. Pat. No. 5,229,102 reports the production of CO2, and H2 by steam reforming of a hydrocarbon in a catalytic ceramic membrane reformer. In the membrane reactor, H2 and CO2 are generated by passing hydrocarbon and steam into the reactor zone in contact with a steam reforming catalyst, e.g., Ni metal promoted with alkali metal. Hydrogen is removed by permeation (or diffusion) through the membrane increasing the efficiency of the reaction.
Catalytic membrane reactors employing gas-impermeable, ion conducting membranes can, for example, be used for oxidation/reduction reactions. For example, oxygen or an oxygen-containing species (such as NOx or SOx) can be reduced at the reduction surface of a catalytic membrane to oxygen-containing anions which are transported across the membrane to an oxidation surface where they react to oxidize a selected reduced species. Materials used in the membranes in such a reactor conduct oxygen-containing anions. Provision must be made in such reactors for electron conduction to maintain charge neutrality permitting anion conduction through the membrane. Electron conduction has been achieved by the use of external circuits for current flow (U.S. Pat. No. 4,793,004). Electron conductivity has also been achieved by doping oxygen-anion conducting ceramic materials with metal ions to generate a material that conducts electrons and oxygen anions. See, U.S. Pat. Nos. 4,791,079 and 4,827,071.
Alternatively, mixed-conducting composite materials can also be made by mixing an oxygen aniononducting material and an electronically-conducting material to form a multiple phase material that conducts both electrons and anions. A preferred method for obtaining mixed- (or dual-) conducting catalytic membranes is to use a membrane material that inherently conducts both electrons and ions. For example, a number of mixed metal oxide materials can be formed into gas impermeable mixed conducting membranes. See, for example, U.S. Pat. No. 6,033,632 and references cited therein.
The present invention relates to gas-impermeable mixed conducting membranes for use in a variety of catalytic membrane reactions and gas separation applications, which are formed by impregnating a porous electron-conducting matrix with a molten salt electrolyte. Ions mediated through these membranes facilitate gas separation and/or provide reactive species for the generation of desired value-added products in catalytic membrane reactors.
This invention relates to mixed-conducting membranes, i.e., membranes that conduct both ions and electrons, which behave as short-circuited electrochemical cells. The membrane comprises a porous electron-conducting matrix and a molten salt that conducts ions. The electron-conducting matrix is at least in part impregnated with molten salt to provide for ion transport through the membrane. The membrane comprises two external surfaces for contact, respectively, with reagent gas and reactant gas. Ions are transported from one external surface to the other external surface of the membrane. One or both of the external surfaces of the membrane can be catalytic. The external surfaces can be provided with adherent catalyst layers, or a three-dimensional catalyst can be provided in close proximity to one or both of the external membrane surfaces. Ions to be transported are formed at or near one external surface of the membrane in contact with reagent gas, transported through the membrane and released at the other external surface where they may react with reactant gas in contact with that surface.
In specific embodiments, one external surface of the membrane is an oxidizing surface and the other external surface is a reducing surface. The reducing surface is typically contacted with an oxidized or oxygen-containing gas. The oxidizing surface is typically contacted with a reduced gas.
Of particular interest are membranes in which the molten salt conducts certain oxide ions, particularly carbonate ion (CO32xe2x88x92). The membranes of this invention are useful generally for production of value-added products using reactive ions that are mediated through the membrane as reagents to convert lower-value starting materials (e.g., hydrocarbons). The membranes of this invention are also useful for the separation of gases and are of particular use for the separation of carbon dioxide from gas mixtures.
More specifically, the membranes of this invention are useful for the generation of carbonate ion from carbon dioxide- and oxygen-containing gas mixtures. The carbonate ion generated and mediated through the membrane is, in turn, useful as a reagent ion for reaction with reactant gases to produce desired products and particularly for partial oxidation of reduced gases. Carbonate ion can be used for the partial oxidation of a variety of chemical species, including hydrocarbons. Using membranes of this invention, carbon dioxide can be removed from gas mixtures containing carbon dioxide and oxygen, particularly air, by reaction of carbon dioxide and oxygen to give carbonate ion at the membrane reducing surface. The carbonate ion produced is then transported through the membrane to the membrane oxidizing surface.
In a specific embodiment, the carbonate ion generated is used to react with natural gas (methane), other hydrocarbons and mixtures of hydrocarbons (optionally in the presence of steam) to form the liquid fuel precursor synthesis gas (syngas, CO+H2) or mixtures of CO2, CO and H2.
Mixed-conducting membranes of this invention in which the molten salt conducts any anion or cation are of general interest. Membranes impregnated with a molten salt that conducts any reactive ion, either anion or cation, are particularly useful in membrane reactors. Membranes of this invention that conduct halide, nitrate, sulfate, or phosphate anions and those that conduct various cations, such as ammonium ion (H4+), are of particular interest for use in membrane reactors and in gas separation processes.
The mixed-conducting membranes of this invention are substantially gas-impermeable membranes formed from a porous electron-conducting matrix which is at least partially impregnated with an ionically-conducting molten salt which functions as an electrolyte to conduct certain ions. Sufficient molten salt is present in the porous electron-conducting matrix to provide for ion conduction through the membrane. In a specific embodiment, the electron-conducting matrix has a central region of fine porosity and external catalytic regions of coarser porosity on either side of this central region. The porosity of the electron-conducting matrix in the central region is sufficiently fine (or small) to substantially retain the molten salt in that region and to substantially fill pores in that region. The porosity of the membrane in the external (or outer layer) catalytic regions is sufficiently increased over that of the central membrane region so that pores in the external regions can be only partially impregnated with molten salt to establish and maintain a three phase interface of gas phase reactants (e.g., CO2 and O2), molten salt electrolyte (e.g., molten carbonate salt), and electrons (from the electron-conducting matrix) to allow reaction to proceed (See: Prins-Jansen et al. (1996) J. Electrochem. Soc. 143: 1617-1628).
Pore size in the central layer of the electron-conducting matrix is typically on average less than about 1 micron and is preferably on average less than about 0.5 micron. Pore size in the external catalytic regions of the membrane is typically on average greater than about 1 micron and is preferably on average about 2-10 micron. The pores in the external regions of the membrane may have a bimodal distribution of sizes with the majority (greater than about 50%) of the pores sufficiently coarse that molten salt is not retained therein and with a minority (less than about 50%) of the pores sufficiently fine that molten salt is retained therein. In a preferred embodiment, up to about one-third of the pores in the external regions of the membrane are sufficiently fine in size to retain molten salt. The fine pores in the external regions of the membrane may be distributed non-uniformly with a higher amount of finer pores located in proximity to the central fine porosity region of the membrane. The membrane may, for example, be formed from a monolithic electron-conducting matrix having varying porosity with a central fine pore region, an intermediate region having a mixture of fine and coarse pores and an outer catalytic coarse pore region. Alternatively, the membrane may be formed from multiple layers of different porosity to achieve the desired pore size distribution and/or may be formed from layers of different electron-conducting materials of different porosity.
In a specific embodiment, the external regions of the porous membrane function to provide sites for catalytic oxidation and reduction reactions. One external catalytic region can provide a reducing surface when in contact with a gas containing carbon dioxide and oxygen (or other ion source gases) and the other (opposite) external catalytic region can form an oxidizing surface to which ions formed at the reducing surface are transported through the membrane. Reactions of anions, e.g., oxide anions, mediated through the membrane can occur at the oxidizing surface. For example, partial oxidation reactions effected by carbonate can occur at the oxidizing surface of the membrane when it is in contact with a gas containing a reactant gas or a reduced gas (e.g., methane or various hydrocarbons). Reactions of cations, e.g., ammonium ion, mediated through the membrane can occur at the reduction surface. For example, ammonium ion can be generated at the reduction surface of a membrane with concomitant reduction of a reactant gas, e.g. hydrogenation of a reactant gas.
The porous electron-conducting matrix of the membrane can be formed from various metals or from electron-conductive ceramics, such as lithium strontium manganate (LSM). Ceramic materials which conduct electrons, but exhibit little or no ion conduction at operating temperature of the membrane are preferred. Various electron-conducting ceramics suitable for use as matrices in the membranes of this invention are known in the art and are readily available.
The membranes of this invention conduct electrons and do not require external circuits to maintain charge neutrality when ions are mediated across the membrane. Thus, membranes of this invention do not require collector plates or similar electron-conducting elements that are employed in molten salt, electrochemical and fuel cells.
Metals useful for forming the electrolyte matrix are transition metals or mixtures thereof. Preferred metals for the porous matrix are nickel and mixtures of nickel with other transition metals. A preferred metal mixture is a mixture of nickel and chromium. Regions of the porous metal matrix of the membrane may react with components of the molten salt or gas components during operation (e.g., during exposure to reactant or reagent gases). For example, regions of the metal matrix in contact with oxygen (or air) may be converted into metal oxide and portions of the metal matrix may react with the molten salt, e.g., the metal matrix may become lithiated when Li molten salts are employed. Matrix material can be formed into a variety of shapes including disks, plates, rings, or tubes (e.g., open-ended tubes or those having one closed end).
Molten salt electrolytes are selected for transport of a selected ion, e.g., molten carbonate salts are employed for carbonate ion transport. Membranes of this invention can be selected for transport of either anions, such as CO32xe2x88x92, or cations, such as NH4+. The molten carbonate used in membranes of this invention is preferably a carbonate of an alkali metal or a mixture of alkali metal carbonates. More preferred molten carbonates are lithium carbonates, sodium carbonates, potassium carbonates or mixtures thereof. Molten halides can be employed for halide ion mediation, e.g., alkaline halides, such as lithium halide or potassium halide or alkaline metal halides, such as lithium aluminum halides. Molten sulfate salts and nitrate salts can be used for sulfate and nitrate ion mediation, respectively. Molten phosphate salts can be used for phosphate ion mediation. Molten ammonium salts can be employed for ammonium cation mediation. Preferred molten salts for use in the membranes of this invention have melting points below about 500xc2x0 C.
Gas-impermeable membranes of this invention can optionally be provided with adherent catalyst layers at either or both external surfaces to promote desired reactions. For example, adherent catalyst layers can be provided at the oxidation surface, the reduction surface or both to facilitate desired reactions. For applications to partial oxidation by oxide ions, an oxidation catalyst layer can be provided on the oxidation surface of the membrane. Particularly in applications to synthesis gas production at the oxidation surface of membranes of this invention, a reforming catalyst, particularly a steam reforming catalyst layer can be provided at the oxidation surface of the membrane. A variety of reforming catalysts are known in the art including various metals (e.g., Ni, Ni-based alloys (e.g., Nixe2x80x94Al or Nixe2x80x94Cr), Co, Pt, Rh, Ru, Pd and mixtures thereof or noble metals catalysts) on various supports, including, e.g., alkaline metal oxides, alkaline earth metal oxides, silica, titania, zirconia, yttria, and mixtures thereof. Ni (about 5-about 10 weight %) on alumina is a preferred catalyst. Catalysts can also be provided as a separate three-dimensional catalyst (e.g., particles or granules) in close proximity (including in contact with) one or both of the external surfaces of the membrane.
A catalytic membrane reactor of this invention comprises a mixed conducting membrane of this invention separating a reagent zone (where the ion to be transported is formed) and a reactant zone (where the ion transported is released for reaction). In a specific embodiment, the catalytic membrane reactor comprises an oxidation zone in contact with the oxidation surface of the membrane and a reduction zone in contact with the reducing surface of the membrane. In one embodiment where the reactive ions are anions, the reduction zone receives a gas mixture containing source gases for the reactive ion to be generated (i.e., a reagent gas) and the oxidation zone receives the reactant gas (e.g., a reduced gas). In another embodiment where the reactive ions are cations, the oxidation zone receives a gas mixture containing source gases for the reactive ion to be generated (i.e., a reagent gas) and the reduction zone receives the reactant gas (i.e., an oxidized gas). A reactor is also provided with appropriate gas inlets and gas outlets (e.g., a gas manifold) for gas handling, including source and reactant gas delivery and product gas collection. The reactor can also be provided with appropriate heating elements to initially heat the membrane to operating temperature in the range of about 500xc2x0 C. to about 800xc2x0 C. The membrane is heated to a temperature that is at least sufficiently high to melt the electrolyte salt in the membrane. Operating temperature may be increased above the salt melting point to ensure efficient operation of the membrane and to optimize oxidation and/or reduction reactions. Operating temperature of the membrane may be, at least in part, maintained by heat released from reactions occurring at the membrane surfaces. The reactor can be formed by of positioning a membrane with appropriate seals between two chambers having appropriate gas inlets and outlets. A reactor contains one or more membranes of the invention and may be comprised of a stack of membranes separated by zones for introduction gases of and/or catalysts.
In a preferred embodiment, the catalytic membrane reactor of this invention is provided with a three-dimensional catalyst in the reagent zone, the reactant zone, or both of the reactor. In a specific embodiment, a three-dimensional catalyst can be provided in the oxidation zone or reduction zone of a reactor in close contact with the oxidation surface or the reduction surface, respectively, of the membrane. In a specific embodiment, for application to the generation of synthesis gas using carbonate mediating membranes, a three-dimensional reforming catalyst is provided in the oxidation zone of the reactor in contact with the oxidation surface of the membrane. For example, a steam reforming catalyst, as is known in the art, can be provided as a three-dimensional catalyst, e.g., as particles or pellets, in the oxidation zone of the reactor in contact with the oxidation surface of the membrane to facilitate synthesis gas production in the catalytic membrane reactor. A reactor of this invention may incorporate both an adherent catalyst layer as well as a three-dimensional catalyst.
In one embodiment, a reactor of this invention is used to separate carbon dioxide from a mixture of gases containing carbon dioxide and oxygen (e.g., air) to generate carbonate ion. This mixture of gases is introduced into the reduction zone of the reactor. The membrane is heated to operating temperatures and carbon dioxide and oxygen are reduced at the reduction surface to form carbonate ion, which is then transported through the membrane to the oxidation surface of the membrane. Carbonate ion transported to the oxidation surface of the membrane during membrane operation can be reacted with one or more gaseous reactants introduced into the oxidation zone. Electrons released at the oxidation surface are transported back across the membrane to maintain charge neutrality. Of particular interest is the use of carbonate to partially oxidize a reduced gas introduced into the oxidation zones. A variety of organic and inorganic species can be oxidized by reaction with carbonate. For example, methane can be partially oxidized to give synthesis gas (a mixture of CO and hydrogen). Alternatively, a variety of hydrocarbons (alkane, alkene, alkynes and aromatics and mixtures thereof) can be partially oxidized at the oxidation surface of the reactor by carbonate anion.
The membranes of this invention containing molten halide, phosphate, sulfate or nitrate salts can be used to form and transport halide, phosphate, sulfate or nitrate ions, respectively. Halide ions formed at the reduction surface in the presence of appropriate source gas(es) and mediated through the membrane can effect a variety of useful reactions, including halogenation. Phosphate ion formed at the reduction surface of the membrane in contact with a gas mixture containing phosphine or phosphorous oxides is transported through a porous matrix/molten phosphate membrane to its oxidizing surface. Phosphate ion can then be employed to react with a variety of chemical species to produce value added product, e.g., organic phosphates. Sulfate ion formed at the reduction surface of the membrane in contact with a gas mixture containing sulfur dioxide and oxygen is transported through a porous matrix/molten sulfate membrane to its oxidizing surface. Sulfate ion can then be employed to react with a variety of chemical species to produce value added products, e.g., organic sulfates. Similarly, nitrate ion formed at the reduction surface of the membrane in contact with a gas mixture containing oxides of nitrogen and oxygen is transported through a porous matrix/molten nitrate membrane to the oxidizing surface. Nitrate ion can then be employed to react with a variety of chemical species to produce value-added products, e.g., organic nitrates
Membranes of this invention containing molten ammonium salts (NH4+Xxe2x88x92 salts, where Xxe2x88x92 is an appropriate anion) can be used to form ammonium ions at the oxidation surface of the membrane and transport the ions to the reducing surface of the membrane. The transported ions can with reactant gases, including oxidized gases, to generate value-added products. Ammonium ion can be generated at the oxidation surface by known electrochemical reactions from ion source gases, for example, from ammonia and steam.